As background to the invention, a general discussion of metal preparation for powder coating will be presented. This will lead into a comparison of conventional metal preparaton systems and the system of the present invention.
One skilled in the art would recognize that the prior art teaches that to achieve a goal of providing the ultimate corrosion protection of steel with an organic powder coating requires several steps be satisfied. First, the surface must be thoroughly cleaned of all dirt, oil, oxidation products and any other foreign matter. Second, sites to which the powder can bond must be available on the surface. These are generallv provided by depositing certain crystals on the surface during the preparation operations and/or by mechanically or chemically roughening the surface. Third, the coating must be specially formulated to impart corrosion resistant properties to the steel when applied in thin coatings generally less than 0.010 inches (0.25 mm).
Those expert in the area of such powder coatings recommend they be applied over mild or "black" steel. Sophisticated surface preparation systems for mild steel have been developed which typically include first cleaning the steel and sometimes roughening it, then rinsing it with a solution which deposits a microscopic layer of crystalline material, such as zinc phosphate. The purpose of this microscopic layer is to passivate the surface against corrosion and to provide bonding sites for the functional powder. Hot dip or electroplated galvanized steel (henceforth referred to as galvanized steel) normally is not recommended as the substrate, despite the superior resistance to corrosion provided by the zinc cladding, because experience has indicated that the organic powders do not bond as well to the inherently smooth zinc cladding as to properly prepared mild steel.
This is demonstrated in U.S. Pat. No. 3,674,445 (Wlodek) which teaches that both nonpretreated and conventionally pretreated hot dip or electroplated galvanized steel surfaces present adhesion problems. Wlodek found that an acceptable bond could be obtained only through the use of vacuum-vapor deposited zinc as a substrate, an exotic plating technique, not available commercially, which produces microscopically rough surfaces as compared to galvanized steel. As a result of this and similar investigations, it appears to have been concluded generally by those skilled in the art that galvanized steel does not constitute a viable substrate for achieving good adhesion of the organic powder coating to the substrate.
The difficulty in obtaining good coating adhesion with a relatively smooth substrate (such as galvanized steel) is demonstrated also by Gemmer (U.S. Pat. No. 3,090,696) and Wamant, et al., (British Pat. No. 815,756), both of which teach that coating adhesion is improved by roughening the surface prior to applying the coating. In fact, Wamant instructs to create "anchor cavities" in the surface to achieve good adherence. Banister (British Pat. No. 1,009,055) recommends blasting the surface to be coated with abrasive particles, such as steel shot, to clean the surface and, while not specifically stated, presumably to roughen it also.
An investigation leading to the subject invention concluded that it was not necessary to roughen galvanized steel prior to coating, as prior art would teach, in order to produce a bond between the coating and galvanized steel equivalent or superior to that obtainable with mild steel substrates. This means that it is possible to utilize galvanized steel as the substrate and thus realize an additional benefit from the superior corrosion resistance of the zinc cladding while still achieving the desired outstanding adhesion of the functional powder coating to the substrate.
A major advantage of the use of galvanized steel over mild steel as the substrate is realized where the powder coating becomes physically damaged, as can occur, for example, in shipping, rigging, or installing industrial equipment. In this situation, the corrosion resistance is determined only by the substrate material and the zinc cladding of the galvanized steel provides significantly greater corrosion protection than can be obtained through the use of a passivating rinse, as typically used on mild steel.
As further background to the invention, the currently used eight-stage or eight-step pretreatment process used by those skilled in the art to prepare hot dip or electroplated galvanized metal prior to coating with an organic powder will be described. The present invention which is an improved four-step pretreatment process will then be described. The eight-stage or eight-step pretreatment process is as follows:
Step 1--The first step cleans the zinc surface of the qaIvanized metal substrate to remove any grease or dirt that is present. This is accomplished by washing or spraying the surface with an alkaline-type cleaning solution with its pH maintained so that it will not attack the zinc.
Step 2--A water rinse is applied to remove the alkaline cleaner from the substrate. Due to carryover of the alkaline cleaner from step 1, this rinse is a mild alkaline rinse.
Step 3--A second water rinse is utilized to remove any alkaline residues remaining on the surface following step 2. Thorough removal of all alkaline residues is important because the fourth step requires a delicate acid balance of a zinc phosphate solution. If any alkalinity is left on the substrate, it will affect the acid balance of the zinc phosphate solution.
Step 4--Substrate passivation, the key to the eight-step system, is accomplished by spraying a zinc phosphate solution on the substrate. This zinc phosphate solution reacts with the zinc substrate to produce and deposit water insoluble zinc phosphate crystals on the surface. It is imoortant that this zinc phosphate solution be maintained at a pH near 3 or a powdery precipitate will be deposited on the substrate. This precipitate is undesirable, as it will significantly reduce coating adhesion.
The zinc phosohate crystals formed on the surface of the substrate serve a two-fold purpose: to passivate the substrate and thereby provide some degree of corrosion protection and to provide irregular molecular sites to which the powder coating can mechanically bond.
Step 5--The fifth step is a water rinse which is necessary to remove excess zinc phosphate solution and any water soluble salts (chlorides, sulfates, or nitrates) that may be on the surface of the substrate. These water soluble salts should be removed from the surface or they will reduce adhesion of the coating.
Step 6--The sixth step is an acidified rinse using chromium compounds such as chromic acid. The primary purpose of this rinse is to remove the less soluble salts remaining after the water rinse in step 5. The chromium compound also deposits an additional barrier coat to give the substrate some added corrosion protection as well as filling some of the pores which exist in the zinc phosphate crystal film, thereby enhancing the passivation of the metal while providing additional molecular bonding sites for the powder coating.
Step 7--The seventh step of the pretreatment process is a water rinse whose purpose is to remove any foreign salts or minerals.
Step 8--The last step involves thoroughly drying the galvanized metal by the application of heat.
It should also be mentioned that in the prior art and in previous pretreatment processes for galvanized metal there is a so-called six-stage or six-step system. This system is similar to the eight-step system except that steps 3 and 7, which are respectively the two water rinse steps, are eliminated. Likewise, step four may also involve iron phosphate in place of zinc phosphate.